A. Field of the Invention
The present invention is concerned with the ultraviolet polymerization of photopolymerizable binders and vehicles, which polymerization is enhanced considerably by the incorporation of certain sensitizers into said binders.
B. Description of the Prior Art
It is known that the photopolymerization of ethylenically unsaturated monomers can be initiated by exposure to various sources of radiation such as ultraviolet radiation. For example, methyl acrylate on long standing in sunlight will generally transform into a transparent mass. Use of sunlight or sources of comparable energy to photopolymerize monomers or combination of monomers, oligomers, etc., is not practical because of very slow rates of polymerization. Improved rates can be induced by applying thermal energy, particularly in combination with free radical promoting catalysts. Thermal energy or heat for this purpose has been supplied by conventional convection ovens and radiant heat from sources such as infrared lamps to effect the desired rate of polymerization. However, for many applications and especially in the coating arts, heat for curing purposes is no longer satisfactory because (1) it is still slow; (2) it cannot be used with heat-sensitive substrates; and (3) it often requires use of a volatile solvent which can be a potential air pollutant and costly to recycle.
To enhance the rate of polymerization of ethylenically unsaturated organic compounds by exposure to radiation such as ultraviolet, photoinitiators or photosensitizers are included with said organic compounds. For clarity, the term photopolymerizable composition as understood herein refers to that composition which hardens (cures) upon exposure to radiation and which can be a vehicle or a binder for use in surface coatings such as paint, varnish, enamel, lacquer, stain, or ink. Typical photosensitizers have been benzenesulfonyl chloride, p-toluenesulfonyl chloride, naphthalenesulfonyl chloride, zinc and cadium sulfides, and sulfinic compounds.
Typical sources for ultraviolet radiation include a number of commercial units such as electric arc lamps, plasma arc torch (see U.S. Pat. No. 3,364,387) and even lasers having a lasing output in the ultraviolet spectrum (see co-pending application of de Souza and Buhoveckey Ser. No. 189,254). The subject matter of the aforementioned patent applications are incorporated herein by reference.
Advantages of the instant invention over the prior proposals include economical and efficient utilization of ultraviolet energy especially in wavelengths between 3200 and 4000 A to perform "cold" polymerization (curing) of the binders or vehicles at very short exposure times with attendant suppression of losses due to volatilization of components of paint, suppression of discoloration or degradation of resulting deposits which can be generally considered as films, and avoidance of shrinkage and distortion (preservation of dimentional stability) and suppression of degradation of substrate to which the polymerizable vehicle is applied particularly when such substrate is paper, fabric, or plastics.